Quantification of bound water content, interstitial porosity and fracture porosity in the sediments entering the North Sumatra subduction zone from Cation Exchange Capacity and IODP Expedition 362 resistivity data

TL;DR

This study quantifies bound water, interstitial, and fracture porosity in sediments entering the North Sumatra subduction zone using IODP data and chemical analyses, revealing porosity variations and their implications for fluid flow and seismic activity.

Contribution

It introduces a novel integrated approach combining resistivity data and chemical analyses to quantify different porosity types in subduction zone sediments.

Findings

Prefan pelagic unit shows high porosity linked to water-bearing minerals.

Interstitial porosity indicates normal sediment compaction without underconsolidation.

Water-rich minerals contribute to porosity anomalies rather than excess pore pressure.

Abstract

In this study, we investigate porosity evolution through the sedimentary input section of the North Sumatra Subduction zone by quantifying interstitial porosity, bound water content and fracture porosity based on IODP Expedition 362 data and post-cruise chemical analyses. During IODP Expedition 362, total porosity of the sedimentary section entering the North Sumatra subduction zone was measured. This total porosity is derived from the total water content of core samples thus including pore water and water bound to hydrous minerals like smectite. Clay mineral composition varies over the sedimentary section and is mainly kaolinite/illite in the Nicobar Fan units and smectite/illite in the prefan pelagic unit below. The prefan pelagic unit shows anomalously high total porosity values and is stratigraphically correlated to a high amplitude negative polarity (HANP) seismic reflector located landward. This HANP reflector has been previously interpreted as a porous fluid-rich layer where the d{\'e}collement may develop along parts of the margin as a consequence of pore pressure buildup. We estimate clay bound water content from Cation Exchange Capacity (CEC) which gives information about the smectite/illite composition and soluble chloride content data. Interstitial porosity corresponds to onboard total porosity corrected from clay bound water and is more relevant in terms of sediment compaction state and fluid flow properties. Interstitial porosity versus vertical effective stress curve shows no evidence of undercompaction and suggests that the input section 2 has been experiencing normal consolidation due to high sediment accumulation rate. The porosity anomaly observed in the prefan pelagic unit results from the local occurrence of water-bearing minerals like smectite rather than excess pore pressure, which might, however, buildup more landward in the basin. We also estimate fracture porosity using a resistivity model for shales used in previous works based on wireline resistivity log and show that fracture porosity yields 4-6% in damaged parts of the sedimentary section investigated.